1. Field of the Invention
The present invention relates to a method utilized in a wireless communication system and communication device thereof, and more particularly, to a method for handling device to device communication in a wireless communication system and communication device thereof.
2. Description of the Prior Art
A long-term evolution (LTE) system supporting the 3GPP Rel-8 standard and/or the 3GPP Rel-9 standard are developed by the 3rd Generation Partnership Project (3GPP) as a successor of a universal mobile telecommunication system (UMTS) for further enhancing performance of the UMTS to satisfy increasing needs of users. The LTE system includes a new radio interface and a new radio network architecture that provides high data rate, low latency, packet optimization, and improved system capacity and coverage. In the LTE system, a radio access network known as an evolved universal terrestrial radio access network (E-UTRAN) includes multiple evolved Node-Bs (eNEs) for communicating with multiple user equipments (UEs), and communicating with a core network including a mobility management entity (MME), a serving gateway, etc., for Non-Access Stratum (NAS) control.
An LTE-advanced (LTE-A) system, as its name implies, is an evolution of the LTE system. The LTE-A system targets faster switching between power states, improves performance at the coverage edge of an eNB, and includes advanced techniques, such as carrier aggregation (CA), coordinated multipoint transmission/reception (CoMP), uplink (UL) multiple-input multiple-output (MIMO), etc. For a UE and an eNB to communicate with each other in the LTE-A system, the UE and the eNB must support standards developed for the LTE-A system, such as the 3GPP Rel-10 standard or later versions.
Please refer to FIG. 1, which is a schematic diagram of a wireless communication system 10 according to the prior art. The wireless communication system includes a network 100 and two UEs 102 and 104. The network 100 may include one or more eNBs 1002 and 1004 which connect the UEs 102 and 104, respectively. In an LTE or LTE-A system, when two UEs communicate with each other, their data path (user plane) goes via the network, even if the two UEs are in close proximity. For example, when the UE 102 needs to transmit data to the UE 104, the UE 102 sends data via the network 100 and the UE 104 receives data via the network 100. In other words, the network 100 receives data from the UE 102 and transmits data to the UE 104. Such procedure is similar to forwarding operation. The typical data path for this type of communication is shown in FIG. 1, where eNB(s) (e.g. eNBs 1002 and 1004) and/or gateways (e.g. serving gateway/packet data network gateway (SGW/PGW) 1000) are involved. UEs which perform this type of communication are said to be in a UE-to-eNB (i.e. UE-to-network) communication mode. However, when the UE 102 and the UE 104 are in close proximity, forwarding data through the network 100 is unnecessary, which wastes the radio resource and causes signal overhead in the network 100.
The specification of 3GPP Rel-11 defines that if UEs are in proximity of each other, they may be able to use a direct data path or a locally-routed data path to communicate with each other. The UEs which perform this type of communication, i.e. a Proximity-based Services (ProSe) communication or a device to device communication, are said to be in a ProSe communication mode or a UE-to-UE communication mode. In the direct data path, the user plane data between the UEs is not traversing any network node. Examples of the direct data path and the locally-routed data path are shown in FIG. 2 and FIG. 3, respectively.
However, there are some issues regarding the ProSe communication in the prior art. First, for a first UE communicating with a second UE each other via a ProSe communication in a data path and/or a control path and having a Radio Resource Control (RRC) connection to an eNB, the first UE may lose communication with the second UE due to radio link problems in the ProSe communication. These radio link problems may happen when, for example, the first UE moves out of transmission coverage of the second UE, or the first UE does not detect any transmission from the second UE. In these situations, the second UE may keep transmitting to the first UE without being aware of this radio link problem and thus drain unnecessary battery power.
Secondly, when a first UE is configured to perform a ProSe communication with a second UE and the first UE has an RRC connection with an eNB, the first UE may encounter a radio link failure or lose communication with the eNB while the first UE still has good radio link with the second UE. In this situation, the first UE may keep communicating with the second UE without the control of the eNB, which causes the first UE to waste power. In addition, the first UE may keep monitoring signal sent by the second UE but cannot receive scheduling command (e.g. uplink grant) from the eNB, which also causes the first UE to waste power. Moreover, the prior art does not deal with the ProSe communication when the first UE recovers the radio link with the eNB.
Thus, how to handle the device to device communication appropriately when a radio link problem occurs is a topic to be addressed and discussed in the industry.